Various optical measurement methods based on active or passive triangulation are used for teeth: phase-measuring triangulation of a strip pattern, as is described in e.g. US patent documents: M. B. Werner H. Moermann, “Method and apparatus for the fabrication of custom-shaped implants”, U.S. Pat. No. 4,575,805, 1984 and G.E. Company, “Non-contact measurement of surface profile”, U.S. Pat. No. 4,349,277, 1980. Other types of strip projections are described in: S.D.G. Systems “3D-Kamera zur Erfassung von Oberflächenstrukturen [3D camera for detecting surface structures]”, DE 4027328, 1990;
K.& V. GmbH, “Optische Sonde zur absolutes 3-dimensionalen Vermessung von Einzelzähnen und Zahngruppen in der Mundhöhle [Optical probe for absolute 3-dimensional measurement of individual teeth and teeth groups in the oral cavity]”, DE 3933994, 1989;
I. G. f. Sondermaschinenbau, “Verfahren und Vorrichtung zur berührungslosen Messung von Gestaltsabweichungen an Oberflächen [Method and device for contactless measurement of figure deviations on surfaces]”, DE 3919893, 1989;
P. D. 6. R. D. Heitlinger and F. 6. W. D. Rödder “Verfahren zur Herstellung von Zahnersatz und Vorrichtung zur Durchfëhrung des Verfahrens [Method for producing tooth replacement and device for carrying out the method]”, DE 2936847, 1979;
A. D. Becker Dental-Labor GmbH, “Verfahren und Vorrichtung zur Herstellung eines Kronenteiles [Method and device for producing a crown part]”. DE 3003435, 1980; and
“Method of three-dimensional measurement with few projected patterns”, U.S. Pat. No. 4,648,717, 1985.
Passive stereo methods are described in N. D. Steinbichler Optotechnik GmbH, “Vorrichtung zur Ermittlung der 3D-Koordinaten eines Objekts, insbesondere eines Zahns [Device for establishing the 3-D coordinates of an object, in particular of a tooth]”, DE 102007060263, 2007.
Active stereo methods, active and passive multi-baseline-stereo methods are described in: B. M. Gregory, D. Hager and E. L. Wegbreit, “Acquisition of three-dimensional images by an active stereo technique using locally unique patterns”, U.S. Pat. No. 7,103,212, 2003 and M. N. Solid Photography Inc, “Arrangement for sensing the geometric characteristics of an object”, U.S. Pat. No. 4,175,862, 1975.
A passive multi-baseline-stereo as a result of a rotating aperture stop is described in F. Frigerio “3-dimensional surface imaging using active wavefront scanning” US 1995/2006.
Active or passive methods for triangulation are distinguished according to whether a pattern to be evaluated, irrespective of type, is projected onto the measurement object (active method) or not (passive method). The name “triangulation” is based on the fact that the camera, the projector and the point of reflection on the measurement object form a triangle.
Use is likewise made of the confocal measurement method. Unlike the aforementioned methods, this is based on the analysis of the local image sharpness or the local contrast of a light point projected onto the measurement object by means of an optical system with a large aperture, as described in “Method and arrangement for a rapid and robust chromatic confocal 3D measurement technique”, U.S. Pat. No. 7,787,132-B2, 2007 and S. D. S. GmbH, “Measuring device and method that operates according to the basic principles of confocal microscopy”, U.S. Pat. No. 7,679,723-B2, 2005.
The aforementioned optical measurement methods generally comprise an illumination system, which conveys light onto the teeth, and an observation system, which analyzes the light after the interaction thereof with the tooth, with the aid of a computer and evaluation unit.
In contrast to most technical measurement objects, human teeth have a pronounced translucence.
The German patent “Verfahren and Vorrichtung zur optischen Erfassung von Oberflächenstrukturen an Zähnen [Method and device for optically detecting surface structures on teeth]”, DE 40 34 007 C2, 1990, discloses the practice of coating the teeth with a fluorescent dye, in particular fluorescein, in order to avoid the problem of translucence. (A)
Coating the teeth with a scattering material, e.g. titanium dioxide powder, constitutes another conventional method for avoiding translucence, as described in the aforementioned document in column 1, line 37. (B)
DE 102007058590-B4 discloses the practice of illuminating the teeth using a thin light line which is then widened to a strip pattern-like structure as a result of the translucence and using this to carry out the measurement method of strip projection. (C)
A disadvantage of the solutions (A) and (B) is the necessity of preparing the teeth. This step, firstly, is a further work step, which is connected to effort. Secondly, very sensitive areas are coated. The coating means has to be removed again because it would interfere with the adhesive connection between tooth and tooth replacement. The nerve in the tooth cannot be cleansed of the coating means without adversely affecting the nerve. On the other hand, the nerve cannot be excluded from the preparation since the whole geometry has to be detected.